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thru boring a 6" chuck for direct bolting to rotary table

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  • thru boring a 6" chuck for direct bolting to rotary table

    Objectively this should be a simple enough. I'm probably over complicating it because drilling out a big new drill chuck seems subversive. But here's the deal- I have a 3 slot Vertex 8" Rotab on a Rockwell knee mill. I found a Shars 6" 4 jaw independent chuck w/o any existing thru bores so I plan to drill three holes offset 120 degrees. I'll drill it on my mill. First I'll clamp the chuck to the centered Rotab with tie downs and layout my 120 degree marks, then remove the chuck and clamp it to mill table with some MDF wood underneath. I'll drill in from the edge about an inch and use a 3 step process .18/.25/.375 bores. The T slots are 3/8 thread and the major diameter is about .372 leaving about 3 thou slack for dialing in to concentricity. That seems like enough.

    Sorta wanted a reality check before I dive in. Thanks !

  • #2
    That is basically what I did with a 3 jaw chuck. Also made a plate that drops in the center bore of the rotab and centers the chuck within .002. On or off in a couple of seconds.

    Why do you need to dial in a 4 jaw chuck? Won't you just dial in the stock?

    Mike

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    • #3
      my way of doing it would be to get a 8 in round plate and mount the chuck to it then bolt it to the rotab that way i wouldn't have to drill the chuck

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      • #4
        Mike it's all a big mystery to me right now. I got the Rotab for some tricky bolt circles on a drill press awhile back. My plan was to buy a MT3 machinable blank and turn it to .75 and center the table by mating to an R8. Tom's Techniques shows that getting him to .001 which is all I need for now. Then I thought the procedure was to indicate the chuck to center it on the spindle- so that's my thinking as it currently exists (hopeful ignorance). I thought this way I could establish .0 on my DRO and then offset the work as needed.

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        • #5
          Kevin, That was my first thought, but I have a 9" belt driven lathe and making an 8" plate is a fair job altho aluminum would prob work, it's another $60 on top and costs me .5 of Z axis. I decided on this bc it's just simpler and cheaper. I just have to get over my chuck fear ;-)

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          • #6
            Strap/bolt the chuck through the central hole to the rotary table.
            Only need 1 bolt/stud.
            Dial in your 120° spacings. Drill them right on the table, no need
            to dismount and drill separately. No need to step up in drill sizes.
            Must be some urban legend I keep hearing, not sure why it keeps
            coming around.

            -D
            DZER

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            • #7
              Great idea to strap thru center. Also leaving them clamped together, I was forgetting the table slot will be underneath the bit. Appreciate your comments.

              I have heard that you use process drilling with large holes over .5 bc smaller machines are not made for the torque required by large holes. I was thinking of it for this bc chuck is made of semi-steel which I thought might be like a sintered or super hard. I've read its scrap metal thrown in with the iron but yeah- thanks it's .37 I'll just drill it. Really helpful thoughts

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              • #8
                After I drilled the holes through my chuck I counterbored for Allen bolt heads. The bolt head never protrudes above the chuck face.

                Mike

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                • #9
                  Darn Mike just when I was down to one operation! How thick are these things? I have a 6" Cushman from the horse n buggy days on my lathe, it feels solid but when I look at the back of the new chucks especially the 4 jaws it looks like maybe the face is 1" or a 1.5 - but .5 will hide the cap screw - thanks for mentioning

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                  • #10
                    Bill, think of the hole theory this way.
                    Drill a pilot for the web thickness for the next size up drill bit.
                    So if you want to drill a 1" hole, and the web thickness of a
                    1" drill bit is 1/4" (about), then first drill a 1/4" hole. then the 1"
                    hole. The idea is the chisel point under the web is just extreme
                    negative rake, just because that is how the point geometry ends
                    up. Extreme negative point angle does not drill well. Needs a lot
                    of force. Splitting the point improves this geometry a good bit.
                    But proper sized pilot is really the way to go. But step drilling in
                    1/16" increments is just not the way to go. It just burns the margins
                    off the bits. Buzz buzz buzz.

                    -Doozer
                    DZER

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                    • #11
                      When I wanted to mount 3 jaw scroll and 4 jaw independent on a 4 tee slot RT, the four jaw just needed front mount holes, bolts and tee nuts. The three jaw problem was solved by mounting the front mount chuck using tapped holes directly into the RT. I should have removed the top to examine it properly, as one of the holes happened to intercept a lubrication drilling which I had not noticed. So a second set of holes were drilled in full thickness metal. Before tightening the three jaw, I gently clamp the jaws onto a MT2 arbor fitted in the centre of the RT. That centralises the chuck and the screws can be tightened. It might be possible to find a position for 4 threaded holes in the top of a 3 slot RT to mount a 4 jaw independent directly. If the threaded holes are no closer than 1/8" from a tee slot, it would be feasible.
                      Drilling the chuck with three holes will result in 2 of them being 15 degrees from the centreline of 2 of the jaws. The further out the holes are drilled results in that 15 degrees being a larger measurement. If the holes in the chuck are drilled on the 5" diameter, 2 of the holes will be 0.654" from the jaw centreline. That may be enough with a 6" chuck.
                      Last edited by old mart; 04-30-2021, 04:05 PM.

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                      • #12
                        I wrote this before the rest of the discussion but got sidetracked, so I'm gonna post it anyway as is:

                        You might find this video useful: https://www.youtube.com/watch?v=MGCBrhsUVu0

                        At any rate, it's not that critical. Especially if you're going to bump it in anyway. In fact, I would go 1/64th over. Working overly-tight when there is no reason for it does nobody any favors. I guess since it's a 4 jaw it doesn't matter if the body runs out, whereas it does on a 3 jaw.

                        PS, have you checked butting 3 caps screws in a 4 jaw? 4 and 3 don't generally jive, but maybe you can fit them in.

                        Lastly, what's with the insane step drilling? 3/8" is a pilot drill in cast iron. Hell, 5/8" is a pilot drill in cast iron unless your tools are small. 3/16" just means a lot more chip clearing and chances for wander.
                        21" Royersford Excelsior CamelBack Drillpress Restoration
                        1943 Sidney 16x54 Lathe Restoration

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                        • #13
                          When I did mine I located the three holes a lot closer to the edge of the chuck than your proposed 1". I wanted to get then as close to the outer edge which will help with the forces generated when you tighten those mounting bolts. CI or semi-steel or whatever your chuck body is made from can and will deform under stress. I tried to get the bolts as close to the outer wall of the chuck body as I could.
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

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                          • #14
                            Drill the mounting holes with substantial clearance, .010" per side minimum, these holes do not align the chuck to the center of rotation.

                            If you do not have a lathe mill a pilot that fits the thru hole of the table in place flip it over then mill a diameter that tightly fits the chuck bore, this will center the chuck whilst the bolts merely hold it down.

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                            • #15
                              Doozer,

                              Thanks for the insight. I went and researched twist drill anatomy to try to understand the "web" and I'm taking it to be the drilll core, the bottom extremity being the chisel edge. When a sequential process is used you end up spinning the chisel edge in the air while forcing the "secondary cutting edge" into the meat. I still dont quite see why the .25 is helping the 1" bit, but that's ok I'll take your word for it and size the web with a caliper for challenging drilling operations. What bothers me with drilling, especially in delrin is having it wander and losing my dimension on the exit hole. THAT is why I started playing around with sequential stuff. I'm one man in a home shop with good tools that feels like he takes the short bus to school every day.


                              Originally posted by Doozer View Post
                              Bill, think of the hole theory this way.
                              Drill a pilot for the web thickness for the next size up drill bit.
                              So if you want to drill a 1" hole, and the web thickness of a
                              1" drill bit is 1/4" (about), then first drill a 1/4" hole. then the 1"
                              hole. The idea is the chisel point under the web is just extreme
                              negative rake, just because that is how the point geometry ends
                              up. Extreme negative point angle does not drill well. Needs a lot
                              of force. Splitting the point improves this geometry a good bit.
                              But proper sized pilot is really the way to go. But step drilling in
                              1/16" increments is just not the way to go. It just burns the margins
                              off the bits. Buzz buzz buzz.

                              -Doozer

                              Comment

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